1. Field of the Invention
This invention relates generally to tube-type audio amplifiers, and more specifically to a high or low R.M.S. output power selection apparatus with no solid state devices employed.
2. Description of the Prior Art
It would be very advantageous to be able to control the maximum R.M.S. output power of a push pull tube type guitar audio amplifier output section. Attempts have been made in the past to achieve this goal. In the previous art, however, there were certain shortcomings associated with the topology of these attempts. Those shortcomings include the use of transistors to regulate the voltage supply to the screen grids of the pentode or beam power type output tubes, and also varying the control grid DC voltage. The use of such transistors destroys the claim of the amplifier being an "all tube" type device. Many audio purists and guitarists shun amplifiers which employ transistors anywhere in the signal path. And while some would argue that transistorized screen grid regulation is not in the signal path, this seems to be a matter of conjecture. Nevertheless, purists do not seem to care about the finer points of this transistor placement argument; if there is a transistor on the amplifier chassis anywhere, it is not an "all tube" amplifier in their eyes.
Another power limiting approach is the use of large, high powered potentiometers to control the screen grid voltage in pentode or beam power output tubes. However, considering the fact that the screen grid of such output tubes can dissipate up to seven or more watts, if four output tubes are employed (which is common), at least a thirty watt potentiometer would be required to sustain normal power dissipations, and if the potentiometer is to withstand the large peak currents which can flow during output tube malfunctions and failures, somewhere in the vicinity of a one hundred watt potentiometer might be needed. This is a large and expensive component, and would take up much valuable space in the chassis. For these reasons, the large potentiometer approach is unattractive.